Abstract

We present tectonic reconstructions and
an accompanying animation of deformation
across the North America–Pacific plate
boundary since 36 Ma. Intraplate deformation
of southwestern North America was
obtained through synthesis of kinematic
data (amount, timing, and direction of displacement)
along three main transects
through the northern (40°N), central (36°N–
37°N), and southern (34°N) portions of the
Basin and Range province. We combined
these transects with first-order plate boundary
constraints from the San Andreas fault
and other areas west of the Basin and
Range. Extension and strike-slip deformation
in all areas were sequentially restored
over 2 m.y. (0–18 Ma) to 6 m.y. (18–36 Ma)
time intervals using a script written for the
ArcGIS program. Regions where the kinematics
are known constrain adjacent areas
where the kinematics are not well defined.
The process of sequential restoration highlighted
misalignments, overlaps, or large
gaps in each incremental step, particularly
in the areas between data transects, which
remain problematic. Hence, the value of the
reconstructions lies primarily in highlighting
questions that might not otherwise be
recognized, and thus they should be viewed
more as a tool for investigation than as a
final product.
The new sequential reconstructions show
that compatible slip along the entire northsouth
extent of the inland right-lateral
shear zone from the Gulf of California to
the northern Walker Lane is supported by
available data and that the east limit of active
shear has migrated westward with respect
to North America since ca. 10 Ma.
The reconstructions also highlight new
problems regarding strain-compatible extension
east and west of the Sierra Nevada–
Great Valley block and strain-compatible
deformation between southern Arizona and
the Mexican Basin and Range. Our results
show ~235 km of extension oriented
~N78°W in both the northern (50% extension)
and central (200% extension) parts of
the Basin and Range. Following the initiation
of east-west to southwest-northeast extension
at 15–25 Ma (depending on longitude),
a significant portion of right-lateral
shear associated with the growing Pacific–
North America transform jumped into the
continent at 10–12 Ma, totaling ~100 km
oriented N25°W, for an average of ~1 cm/yr since that time.